1. The Field of the Invention
The present invention relates generally to fuel cells. More particularly, the present invention relates to carbon nanorings as an electrocatalyst support for fuel cell applications.
2. The Relevant Technology
Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy. A common type of fuel cell is the hydrogen fuel cell, which generates energy through a controlled reaction between hydrogen and oxygen. Recent improvements in fuel cells have led to the development of other types of fuel cells, such as direct methanol fuel cells (DMFCs) and direct ethanol fuel cells (DEFCs).
Essentially all fuel cells require a catalyst to increase the rate of the reaction in the fuel cell. Fuel cell catalysts are typically supported on a support material such as carbon black. Currently, most of the best performing fuel cell catalysts are supported on carbon black (Vulcan XC-72R), which is used for its reasonable surface area and number of mesopores.
Reaction rates in heterogeneous catalysts usually depend on five fundamental steps: (i) outer and inner diffusion of reactants, (ii) reactant adsorption, (iii) reaction on the active site (iv) desorption from the active site, and (v) product release from the catalyst. In conventional catalysts, steps (ii)-(v) have been rate determining steps. Improvements in steps (ii)-(v) can be obtained by improving the number of active sites on the catalyst. Improving the number of active sites can be accomplished without using more metal by reducing the size of the catalyst particles. Consequently, much effort has been made for reducing the size of catalyst particles.
In some cases, advancements in reducing the particle size have improved steps (ii)-(v) to such an extent that the diffusion of reactants (i.e., step (i)) is the rate limiting step. Attempts have been made to improve the performance of these catalysts by selecting new support materials that can improve mass transfer and/or electron transfer to the catalyst surface. New developments in carbon nanomaterials present a possible improvement in support materials over carbon black. For example, single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanohorns, carbon nanocoils, and ordered meso-porous carbon have been investigated recently for their potential use as supports for fuel cell catalysts. Of these catalysts, the catalysts supported on carbon nanohoms and carbon nanocoils have exhibited higher performance than catalysts supported on carbon black.